1. Field of the Invention
The invention relates to a circuit protection apparatus, and more particularly to an over-current protection apparatus.
2. Description of Related Art
In recent years, electronic devices have achieved higher precision and complexity so that a stable power source becomes more important. Take the power source for the notebook computer for example, the arrangement of the power source includes an internal rechargeable battery and a connection to an external power supply. This arrangement allows the notebook computer to use the public electric power as the power source and, at the same time, to charge the rechargeable battery for use when no public electric power is available to the notebook computer.
Referring to FIG. 1, a conventional approach to supplying power to an electronic device is schematically illustrated. Take a notebook computer as an example of the electronic device. A power supply 11 is an external power source (other than the rechargeable battery) for the notebook computer. The main functional circuit 12 represents the circuit for implementing the function of the notebook computer. The power supply 11 is coupled to the main functional circuit 12 via the electronic switch and so as to supply the electric power to the main functional circuit 12, wherein the electronic switch can be a transistor. The transistor can be a bipolar junction transistor (BJT) or a metal oxide semiconductor field effect transistor (MOSFET or MOS), such as an N-type MOS (NMOS) transistor or P-type MOS (PMOS) transistor. In FIG. 1, the electric switch is an NMOS Q, for example. With respect to the NMOS Q, the source electrode can be used as a power source terminal Vin. When the power supply 11 is connected to the notebook computer, the gate electrode of the NMOS Q can be coupled to a high voltage VH of the power supply 11 via a resistor R, so as to turn on the NMOS Q. In this manner, the main functional circuit 12 can obtain the electric power generated by the power supply 11 via the power source terminal Vin.
When a short circuit occurs inside the computer due to unknown reasons, according to the usual design for the power supply, the power supply 11 will perform automatic power cut-off. Unfortunately, the effective value of the short-circuit current is of a certain amount. Thus, a high temperature would occur inside the computer because the amount of the short-circuit current is too large or the short-circuit current exists for a long time. This would cause damage to the electronic parts or even to the whole main board in the worst case.
When the short circuit occurs inside the computer, all power will be brought down to a low voltage level of about zero. In this case, no power source inside the computer system is available to activate the over current protection circuit. For the current situation, when the notebook computer does not work due to the occurrence of a short circuit, the short-circuit current is typically at least 2 to 4 ampere (A). In particular, the power supply for P4 CPU will consume the electric power of at least 80 watt (W). For this reason, the value of the short-circuit current may be as large as about 5 ampere (A). This large short-circuit current would cause serious damage to the computer system.
The conventional methods to solve the damage of a short circuit fall into two categories. One is to reduce the effective current values generated by the power supply due to the occurrence of the external short circuit. The other is to increase the power and current tolerance of the electronic switching device. With respect to the first method, as the operating frequency of the CPU implemented by the notebook computer grows faster and faster, the power consumption for the computer system will grow accordingly also. Then, the power from the power supply must be at a sufficient level for use. In this situation, the goal of reducing the level of a short-circuit current is not easy to achieve. With respect to the second solution, although the electronic switching device with the capability for enduring the large current and the high power can temporarily prevent the system from being damaged by the high temperature, the short circuit failure is not solved yet and the short-circuit current remains. After a long period, the electronic parts or the main board still could be damaged in the end. This second method only postpones the occurring time of damage, and cannot avoid damage due to the short circuit. Furthermore, the fabrication cost is high for the electronic switching device capable of enduring the large current and the high power, resulting in a high cost of the power protection design.